NOISE GENERATOR 8121C

 Model-8121C uses a logic circuit to generate “Pseudo Random Noise”, it is the newest and the most advanced design to generate audio white noise. It has much flatter frequency response than using diode. Model-8121C has: (1)White noise source.(2)Weighting filters.(3)Built-in power amplifier.(4)” TRUE RMS ” voltage meter circuit. It can generate five different kinds of noise’s spectrum, including White Noise, Pink Noise and three kinds of “Weighted Noise”, conforms to following nation’s standards:(1) JIS C5531, IEEE 219, CNS 4785 (2) EIA 426B(3) DIN 45573 

NOISE GENERATOR 8121C MANUAL:

Model-8121C（ Signal Generator )

The weighting filter 8121FT-1

 Since the Noise Generator (Model-8121C) is mainly designed for loudspeaker testing, thus, it is equipped with a 50W or 100W power amplifier, and a “TRUE RMS” voltage measuring circuit, to measure the “TRUE RMS” of output noise. Never use a general VTVM, AC millivoltmeter, or a digital voltage meter to measure noise voltage, unless when these meters have especially noted that their AC testing function is a TRUE RMS mode, otherwise, the testing result of RMS value is only precise for a sinusoidal signal, because these meters are designed for sinusoidal RMS voltage testing. If these meters are used to test nonsinusoidal signals (including noise signals), the results of measured RMS values will be inaccurate, therefore, the 8121C is equipped with a “TRUE RMS” testing circuit, to measure the noise output voltage. Since noise voltage itself is not highly stable, thus, the voltage meter’s indicator needle will have a slight deflection, don’t mistake this as 8121C was faulted.

 Model-8121C has also provided a selectable “Filtered Noise Sig. Output” terminal (9), which has the following two usages: (1) It can connect externally with another power amplifier ; (2) It can connect externally with another weighting filter, usage of an external weighting filter is: When users or other national standards required ” weighting filter network with the special specification “; then, users can connect (9) to the external needed filter, for necessary filtering, then, connect the output of an external filter back to the built-in power amplifier input terminal (10) of Model-8121C, for power amplifying, thereby measuring the pre-measure

Noise Generator “Model-8121C” has four features for loudspeaker test, which are:(1)Loudspeaker’s continuous load test.(2)Loudspeaker’s rated power handling capacity specifying based on the above testing results.(3)Feeding noise signal to the loudspeaker to measure the sound pressure sensitivity (dB). The following is the details:
1. Loudspeaker’s Continuous Load Test
The IEEE 219 recommends that a signal from a white noise generator which passes through a specified weighting filter can be filtered and then be amplified, for continuous load test. Thereafter, most nations use the same rule for their national standard, for example CNS 4785, JIS C5531 etc. But U.S. EIA 4264A uses another kind of weighting
filter, there are more high frequency than IEEE 219 at this spectrum, it might be because of a wider frequency band in late music. For loudspeaker’s continuous load test, select first the needed noise signal pushbuttons (3) to (7),set the external filter selecting switch (11) to “Without ext. filter” position, then, estimate the rated power handling capacity of pre-measure loudspeaker, to adjust the volume adjusting adjusting knob (12), thereby obtaining the needed testing voltage.
REMARK: If either one of the noise signal push-buttons (3) to (7) is not being pressed in, then, no single noise signal output will occur, it means that the filtered noise signal output terminal (9) will have no output signal, besides no matter the volume adjusting knob is being adjusted to the largest volume, still, no single output will occur from output terminals (2). If the pre-measure loudspeaker is not a full range type (like woofer, midrange or tweeter) and a crossover network is installed, the IEEE recommends that the crossover network should not be taken apart, and should be measured together. Furthermore, if the pre-measure loudspeaker is a single unit without crossover network, then the manufacturer should specify an adequate crossover network for buyers to connect with loudspeaker for this testing, at this time, the total impedance should be measured from the terminals of
crossover network, not from the loudspeaker (noted from IEEE std.) The IEEE recommends the continuous loading period should be 100 hours at a power of specified level, then let the pre-measure loudspeaker rest a non-working condition for another 100 hours. After these procedures, if there is no significant change in the electrical, mechanical and acoustical characteristics of this loudspeaker, the testing is consider qualified. If the testing result is not acceptable, then reduce the noise signal power and fed to the pre-measure loudspeaker, and do the test at the same condition till the loudspeaker can pass the test. On the other hand, if an initial testing result is acceptable,
users should increase the loading power & repeat the same procedures till the premeasure loudspeaker fails to handle such loading power, After these repeating test, themaximum power of a loudspeaker for continuous loading can be defined.
2. Loudspeaker’s Rated Power Capacity Specifying :
Up to the present time, there isn’t any instrument that can indicate a loudspeaker’s rated power capacity immediately after the two terminals are being connected. All these data are the result of continuous load test. According to this value, Users can specify the rated power capacity of a loudspeaker. The IEEE recommends that a loudspeaker must pass continuous loading test at it’s specified rated power capacity.

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